1. Field of the Invention
The invention generally relates to the circuit switched fallback (CSFB), and more particularly, to determine whether the candidate cell belongs to the registered location area (LA) in CSFB process.
2. Description of the Related Art
With growing demand for mobile communications, the Global System for Mobile communications (GSM) supporting only circuit-switched (CS) domain services no longer meets user requirements. Accordingly, the mobile communications working groups and standard groups have developed the so-called third generation mobile communications technologies, such as Wideband Code Division Multiple Access (W-CDMA), Code Division Multiple Access-2000 (CDMA-2000), Time Division-Synchronous Code Division Multiple Access (TD-SCDMA), etc. Take the Universal Mobile Telecommunications System (UMTS) using the W-CDMA technology, for example: the 3rd Generation Partnership Project (3GPP) has further proposed the Long Term Evolution (LTE) system, and called for the fourth generation mobile (4G) communications system to be implemented to meet future demand for large wireless data-transmission bandwidths. The LTE system aims to provide an all-IP architecture, in which only a packet-switched (PS) domain, instead of a CS domain, is used to carry all mobile communications services. Also, voice services are provided by Voice over Internet Protocol (VoIP) in the fourth generation mobile communications system, instead of the CS domain as per the second and third generation (2G/3G) mobile communications systems.
However, during transition from the second and third generation mobile communications systems to the fourth generation mobile communications system, some operators have still chosen to provide voice services via the CS domain of the second and third generation mobile communications systems, due to the coverage rate and capacity of the CS domain of the second and third generation mobile communications system being already sufficient. This raises a problem wherein the architecture of the second and third generation mobile communications systems comprise both the CS domain and PS domain, while the fourth generation mobile communications system comprises only the PS domain. Accordingly, the interconnection between the second and third generation mobile communications systems and the fourth generation mobile communications system, especially the CS domain part, must be defined and specified, so that systems of different standards can achieve smooth interoperability to provide voice services to users. One technique used is the Circuit Switched Fallback (CSFB) architecture, which is defined in the 3GPP TS 23.272 specification.
However, in the CSFB architecture, the UE may need to perform a Location Area Update (LAU) during the CSFB procedure when the location area (LA) of the CSFB target of the second or third generation cell is different from the registered LA. Therefore, this Location Area Update may generate an extra call-setup delay.